The present invention relates to a semiconductor device with an improved multi-layered insulation structure formed on a semiconductor body.
In a semiconductor device, for example, in a bipolar transistor, the main factors which influence the withstand voltage characteristics and the reliability of operation are: the kinds of insulation layers formed in a multi-layered structure on the semiconductor body, the thickness of each insulation layer, the structure of the insulation layers, the blocking performance of each insulation layer against external impurities such as water, the method of formation of the insulation layers and so on.
In order to improve the withstand voltage characteristics and the reliability of operation, many steps have been conventionally taken for decreasing the field strength by making the insulation layer formed on the semiconductor body thicker, for forming a separating region for eliminating generation of channels, for forming an insulation layer of high blocking ability against impurities and so on.
However, when the insulation layer is made thicker, several problems arise; a thick insulation layer makes an aluminum layer on it discontinuous at a thick stepped portion and the blocking ability against external impurities is reduced. When a separation region is formed for preventing the generation of channels, the required chip area is made greater and the packing density is degraded. When an insulation layer of a high blocking ability is formed, the diffusion of impurities introduced from outside into the insulation layers and the semiconductor body may indeed be prevented. However, since an insulation layer of such a high blocking performance has a large amount of charge, an inversion layer is apt to be formed in the surface region of the semiconductor body. This not only increases the leakage current but also results in actuation of parasitic elements. This might eventually lead to a breakdown of the semiconductor device and degradation of the reliability of its operation.